Rolling Bearings as Crime Scenes

How to determine the root cause of bearing failure by analyzing telltale signs.

The calculated life expectancy of any rolling bearing is traditionally grounded in five basic assumptions:

• The bearing receives timely lubrication in the proper type and quantity.
• Mounting is performed properly.
• Bearing dimensions are consistent with specifications for the application.
• The bearing is supplied defect-free.
• Recommended maintenance is performed.

If all these conditions can be met, the only reason for a bearing to fail is from material fatigue associated with shear stresses cyclically applied immediately below the load-carrying surfaces.

Unfortunately, in real-world operations, all these conditions rarely will be satisfied and bearing service life will be threatened regularly by the effects of abrasive wear, corrosion, electrical erosion, fractures or a combination of these or other factors.

When bearings sustain damage and must be replaced, profit loss follows due to equipment downtime and lost productivity. Unless the root cause of failure can be detected, history may repeat itself with more of the same after a new bearing is installed and the rotating machinery resumes operation.

In determining root cause, practical lessons can be learned by inspecting and analyzing a failed bearing, which can be read like a "crime scene." Telltale signs are often apparent.

Classifying Bearing Failures

ISO (International Organization for Standardization) has developed a methodology for classifying bearing failures (ISO Standard 15243-2004E), which recognizes six primary failure modes (related to post-manufacturing sustained damage) and identifies the mechanisms involved in each type of failure.

Referencing this ISO standard, most rolling bearing damage can be linked to the following modes:

• Fatigue defines a change in the bearing's material structure caused by repeated stresses in the contacts between rolling elements and raceways. Subsurface fatigue shows as micro-cracks at a certain depth under the surface, and surface initiated fatigue is flaking that originates at the rolling surfaces.
• Wear is the progressive removal of material from the bearing's sliding or rolling contact surfaces during service. Abrasive wear usually can be linked to inadequate lubrication or ingress of contaminants, and adhesive wear (or smearing) follows transfer of material from one surface to another.
• Corrosion on a bearing can arise from several causes. Moisture corrosion forms corrosion pits from oxidation of the bearing's surface; fretting corrosion (a type of frictional corrosion) is the oxidation and wear of a bearing's surface under oscillating micro-movements; and false brinelling (another type of frictional corrosion) creates shallow depressions resulting from micro-movements of a bearing under cyclic vibrations.
• Electrical erosion occurs when the passage of electric current removes bearing material from contact surfaces. Evidence of excessive voltage can be viewed as electrical pitting on the bearing generated by sparking and localized heating from ineffective insulation, and current leakage will develop shallow craters, or electrical fluting, on a bearing.
• Plastic deformation is a permanent condition when the yield strength of the bearing material is exceeded. Overload (or true brinelling) creates shallow depressions or flutes in the raceways; indents from debris develop when particles are over-rolled; and indents from handling occur when bearing surfaces are dented or gouged by hard, sharp objects.
• Fractures occur when the ultimate tensile strength of bearing material is exceeded, causing a complete separation of a part of the bearing. Forced fractures result from a stress concentration in excess of the bearing material's tensile strength. Fatigue fractures occur when the fatigue strength limit of the material is frequently exceeded. Thermal cracking (or heat cracking) will form cracks due to high frictional heating.

Sleuthing the Scene

Bearing damage analysis affords practical insights into the causes and the condition of the rotating machinery. Whenever possible, evidence from a failed bearing should be collected and interpreted to establish what went wrong and why. Even visual examination and a limited application review can suggest likely causes to avoid costly recurrences.

Definitive signs in "reading" the damage will suggest particular damage mode causes. Some of the usual suspects include ineffective lubrication, ineffective sealing, static vibration or passage of electric current. Look for the following suspects when attempting to ascertain whether one of these causes is culpable:

Suspect #1: Ineffective Lubrication

Lubricant for a rolling bearing separates the rolling elements, cage and raceways in both the rolling and sliding regions of contact. Without effective lubrication, metal-to-metal contact occurs between the rolling elements and the raceways, causing wear of the internal rolling surfaces. Most cases of "lubrication failure" result from insufficient or excessive lubricant viscosity, over-lubrication, contamination of the lubricant or inadequate quantity of lubricant. When lubrication is ineffective, abrasive wear or adhesive wear surface damage can occur.

The first visible indication of trouble due to abrasive wear is usually a fine roughening or waviness of the bearing's surface. Fine cracks can then develop, and spalling (or surface-initiated fatigue) will occur. If there is insufficient heat removal, the temperature may rise high enough to cause discoloration and softening of the hardened bearing steel.

In the case of adhesive wear, a bearing's surface assumes a "frosty" appearance and will feel smooth in one direction but distinctly rough in the other. Or, smearing damage may be perceived.

One type of smearing develops between sliding surfaces, whereby minute pieces of one surface tear away and re-weld to either surface. (Areas subject to sliding friction, such as locating flanges and the ends of rollers in a roller bearing, are usually the first parts to be affected.) Another type of smearing is called "skid-smearing," which can be detected as patches. This condition can result when rolling elements slide as they pass from the unloaded to the loaded zone, and there is insufficient lubrication in the load zone.

Suspect #2: Ineffective Sealing

The effects of contaminants on bearings without proper sealing systems can be devastating, even affecting lubricant performance. When debris is trapped between a bearing's raceway and rollers, plastic deformation depressions, or particle denting, can develop. When spalling debris causes this condition, the effect is known as fragment denting. Each type of these small dents can be viewed as the potential origin of premature fatigue.

In addition to abrasive matter, non-particle corrosive agents can invade. Water, acid and many cleaning agents deteriorate lubricants and lead to corrosion. Acids form in the lubricant in the presence of excessive moisture and detection will show that the bearing's surface is etched black.

Suspect #3: Static Vibration

Bearings do not have to be rotating to be damaged in an application. When vital equipment has a standby back-up unit, damage can be experienced from transient vibrations caused by running machinery. Depending on the proximity of the idle unit to the unit in operation, vibrations generated from the running equipment can cause the rolling elements in the bearing of the static machine to vibrate. These create the condition of false brinelling, or a wearing away of the raceway surface in an oblong or circular shape. When the standby equipment is placed into service, the bearings usually will be noisy and require replacement.

Suspect #4: Passage of Electric Current

Static electricity emanating from charged belts or from manufacturing processes can pass through a machine's shaft and through a bearing to the ground. When this current bridges the lubrication film between bearing rolling elements and raceways, microscopic arcing occurs. This produces localized and extreme temperatures that melt the crossover point. (The overall damage to the bearing will be proportional to the number and size of individual damage points.)

Electrical erosion fluting due to current leakage results when these moderate voltage small currents arc during prolonged periods. Microscopic pits accumulate drastically. Flutes can run deep, exhibit noise and vibration during operation, and lead to eventual fatigue.

Other than the obvious fluting pattern on a bearing's rings and rollers, another sign of current leakage will be indicated by a darkened gray matte discoloration of the rolling elements and a fine darkened gray matte discolored load zone.

Conclusion

Bearing damage analysis ultimately will contribute to the reliability of rotating machinery. Experienced bearing manufacturers can offer the value-added resources to assist in the detective work and distinguish useful information from false or misleading clues.

Pumps and Systems, March 2009